"Wireless cable" is a term usually used to refer to a multi-channel video distribution medium that resembles franchise cable television, but which uses microwave channels rather than coaxial cable or wire to transmit programming to the subscriber. Programming for wireless cable systems is received at the headend of the wireless cable system in the same manner as it is for landline based cable television. These programs are then re-transmitted, utilizing the high end of the Ultra High Frequency (UHF) portion of the microwave radio frequency spectrum (2.1 to 2.7 GHz), by a microwave transmitting antenna located on a tower or other tall structure to small antennas on subscriber rooftops, typically within a 40 mile radius.
At the subscriber's location, microwave signals are received by an antenna, down-converted and passed through conventional coaxial cable to a descrambling converter located on top of a television set. The signals are converted at the antenna location to lower frequencies in order to be carried over conventional in-house cable to a converter box, decoded and then output to a standard television set. Because wireless cable signals are transmitted over the air rather than through underground or above-ground cable networks, wireless systems are less susceptible to outages and are less expensive to operate and maintain than franchise cable systems. Most service problems experienced by wireless cable subscribers are home-specific rather than neighborhood-wide, as is frequently the case with franchise cable systems.
As a general matter, the transmission of wireless frequencies requires clear line-of-sight (LOS) between the transmitter and the receiving antenna. Buildings, dense foliage and topography can cause signal interference which can diminish or block signals. Certain LOS constraints can be reduced by increasing transmission power and using engineering techniques such as pre-amplifiers and signal repeaters.
In a typical prior art system, such as shown in FIG. 1, a headend system H receives a number of analog television program signals from a variety of satellite down-link receivers and other types of receivers, in the exact same manner as for a cable television system. The headend system H frequency multiplexes those television program signals into a combined spectrum signal in the 50-450 Mhz range. This combined signal has a frequency distribution similar to that found on a cable television network. The headend system upconverts the combined spectrum signal to the UHF frequency range, typically centered around 2.6 GHz. The headend system supplies the UHF signal to a single transmitter antenna tower T which broadcasts the signal to subscribers who each have an individual home receiving system. Subscribers can call in to the headend to order pay-per-view events via the telephone network, and the headend transmits codes to the subscribers systems to enable descrambling of encoded pay-per-view programs.
FIG. 1A shows a typical service area for a wireless cable type system of the type shown in FIG. 1. In accord with relevant regulations, the wireless cable operator has a protected or `primary` reception area P. At the relevant frequencies here under consideration, the primary area P is a circle having a radius of 15 miles from the operator's transmitter T. Within this area, the operator is guaranteed that there will be no interference with his transmissions on the assigned frequency channel(s). However, at the allowable power levels, the transmissions from antenna tower T will propagate out over a secondary area S having a radius of up to 40 miles. Within the secondary area, some locations will receive sufficient signal strength to utilize the wireless cable services.
UHF signals in the relevant frequency band arrive at the customer location by direct line-of-sight (LOS) transmission. Typically an elliptical dish shaped antenna 18-36 inches long, formed of parallel curved elements, is aimed from the subscriber location to receive the strongest signal from the transmitter. The captured signals are .down-converted at the antenna from the microwave band to the broadcast band and transmitted via coaxial wiring into the house. For scrambled signals (the typical case), a set top converter functionally similar to a cable set top box is used. In many UHF installations, to conserve UHF capacity for premium services, a VHF/UHF off-air broadcast receive antenna is installed with the UHF antenna to pick up the local programming.
The evolution of wireless cable may be briefly summarized as follows. Wireless cable technology has existed in a single channel version for commercial purposes since the 1970's and had been available even longer for educational use. In mid-1983, the FCC, invoking the need to promote competition with conventional cable television systems, established a change in the rules for using a portion of the microwave spectrum previously designated for educational use. In the past, 28 microwave channels had been available to accredited and non-profit educational organizations for educational use exclusively by Instructional Television Fixed Service (ITFS) operators. The new rules reallocated eight of those channels for outright commercial use, and educational organizations were permitted to lease excess hours on the remaining 20 channels to commercial operators. In any local market, this makes it possible for a commercial operator to combine available time on any or all of those 28 channels with five other channels already available for commercial use. Thus, under the current FCC rules, the available spectrum results in a maximum of 33 analog channels. This number of `wireless cable` channels is less than the number offered on many competing franchise type cable television systems.
Since 1983 spectrum blocks in the 2.1-2.7 GHz range have been allocated for the purpose of delivering video content from a single transmit site to multiple receive locations. A total of 198 Mhz has been allocated for downstream transmissions for the wireless cable service. The channelization and transmission modulation (6 Mhz amplitude modulation/vestigial side band) are equivalent to broadcast TV or cable but up-converted to the assigned microwave frequencies.
The relevant portion of the UHF spectrum was originally licensed in blocks of four video channels each separately licensed, with each block allocated to a specific purpose. Five groups, each with four channels, were allocated to Instructional Television Fixed Service (ITFS). Two groups of four channels were made available to anyone wishing to provide an alternative multi-channel video program service. The final four channels were licensed individually to institutions for the purpose of providing a private video network. Through licensing and leasing arrangements, the FCC now allows all of the channels to be aggregated for the purpose of providing an alternative to cable television.
The 33 channels potentially available to wireless cable operators thus .are subdivided into two types of channels. Twenty channels are referred to as ITFS. The remaining 13 channels are generally referred to as Multi-channel Multipoint Distribution Service (MMDS).
In many ways, current typical UHF wireless TV is equivalent to a low tier franchise cable television system (i.e. having relatively few channels), with the only real difference lying in the medium used to transport signals from the headend to the customer. Functionally identical headend equipment is utilized in both systems. In the case of UHF service, signals leave the headend via a microwave transmitter. With cable television, the same signals leave the headend on fiber or coaxial cable facilities.
Wireless cable technology provides a relatively low cost medium to transmit video and does not require extensive coaxial cable networks, amplifiers and related equipment. The three major advantages of such service are variable cost technology where capital is spent in establishing cash flows, manageable financial risk because of variable costs, and the possibility of establishing broad based market entry more quickly than is feasible with wireline based video systems. Wireless cable systems are attractive to potential subscribers not yet served by franchise cable operators and can provide customers in cabled areas with an economical alternative to both existing franchise cable and satellite television reception systems. However, the current analog technology presents several problems which have severely limited actual use of `wireless cable`.
Propagation characteristics at the relevant UHF operating frequencies require line-of-sight (LOS) between the transmit and receive antennas for reliable service reception. Both natural obstructions such as hills and vegetation, and man-made obstructions such as buildings, water towers and the like, limit the actual households capable of receiving an LOS transmission. FIG. 1A shows a simplified example of one such obstruction O. As illustrated, the obstruction O is within the primary reception area P. The obstruction blocks line-of-sight transmissions from transmitter antenna tower T in a radially extending blockage or shadow area B. Receiving systems within this area can not receive the transmissions from antenna T, and potential customers in that area B can not subscribe to the wireless cable services broadcast from that tower.
One solution to the blockage problem has been to provide repeaters. A repeater receives the primary transmission from tower T on the tower side of the obstruction, amplifies the signal if necessary, and retransmits the signal into the area of blockage. This may be an effective solution to one blockage or obstruction O, but in many major metropolitan areas there are many obstructions. The power levels of such repeaters tend to be low, necessitating use of a large number of repeaters. Also, because of delays and multipath effects, repeater transmissions may interfere with reception from the primary source in areas close to the blockage area B. Overcoming blockages using repeaters together with the necessity for minimizing the attendant distortions that result when amplifying combined RF channels would therefore require an inordinate number of repeaters.
In the industry, a nominal figure for households reachable by LOS is 70%, even with a small, commercially practical number of repeaters. This projected number is based solely on computer models, not actual field measurements. It is believed that actual coverage by the current wireless cable technology in the UHF medium is considerably lower. Typical antenna heights required to achieve the present level of coverage in commercial service are 800-plus feet for transmitters and 30-60 feet for receivers. That means that many receive antennas must be mounted atop masts or nearby trees as an alternative to a rooftop mounting. While current regulations provide a 5 mile protected service area for MMDS, it is desired that effective system coverage for approximately 40-70% of the affected households may be achieved to a 40 mile radius from the transmitter antenna and using relatively low roof-mounted receiving antennae wherever possible.
Besides signal blockage, several other propagation factors can affect reliable UHF service delivery. One factor is multi-path reflections of the desired signal arriving at the receiver by way of differing paths and therefore arriving with slight delay. For analog video signals, multi-path appears as ghost images on the viewer's TV. For digital signals, multi-path can cause intersymbol interference that results in multiple bit errors. In either case, near-coincident multi-path signals can cause a degree of signal cancellation that looks like additional propagation loss. Multi-path also results from reflections and diffraction.
Path fading is another significant coverage factor. Time-variant path fading can result from atmospheric effects, e.g., rain or temperature and pressure inversions. Rain can act to partially reflect or absorb the microwave signals.. Weather inversions can result in an upward bending of the wave front due to refraction. There are engineering measures to mitigate the troublesome effects of time-variant path fading, such as suitable fade margins and antenna diversity.
In the paging and radio communication fields, various systems of sequencing and simulcasting have been proposed to achieve some increased coverage. Examples of typical proposed systems are illustrated in FIGS. 2 and 3. The related systems are described in U.S. Pat. No. 3,836,726, issued September 1974 and U.S. Pat. No. 5,038,403 issued Aug. 6, 1991. FIG. 2 illustrates a system utilizing sequencing while FIG. 3 illustrates a system utilizing simulcasting. As can be seen, the aim is to cover a maximum area with minimum areas of signal overlap. Even if someone suggested application to UHF Wireless Cable type communications, such propagation fields would still exhibit the above noted problems due to obstructions, multi-path interference and fading.
Clearly a need exists for a broadcast system providing increased propagation coverage and reduced areas of blockages. Any such system should also provide an increased number of programs, without requiring additional spectrum allocation. The system should provide good signal quality throughout the entire reception area or service area. Accordingly, it is also desirable to minimize multipath interference and loss of service due to fading.